<!DOCTYPE html>
<html>

<head>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">
    <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.bundle.min.js"></script> </head>

<body>
    <br>
    <br>
    <br>
    <br>
    <br>
    <br>
    <br>
    <br>
    <!-- Default dropup button -->
    <div class="btn-group dropup">
        <button type="button" class="btn btn-secondary dropdown-toggle" 
            data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
            Dropup </button>
        <div class="dropdown-menu">
            <a class="dropdown-item" href="#">Action</a>
            <a class="dropdown-item" href="#">Another action</a>
            <a class="dropdown-item" href="#">Something else here</a>
            <div class="dropdown-divider"></div>
            <a class="dropdown-item" href="#">Separated link</a>
        </div>
    </div>
</body>

</html>